Upgrade to Rails 3.2

In reference to https://github.com/MarkUsProject/Markus/issues/1208

I get the usual deprecation warnings about using vendored plugins:

> DEPRECATION WARNING: You have Rails 2.3-style plugins in vendor/plugins! Support for these plugins will be removed in Rails 4.0. Move them out and bundle them in your Gemfile, or fold them in to your app as lib/myplugin/* and config/initializers/myplugin.rb. See the release notes for more on this: http://weblog.rubyonrails.org/2012/1/4/rails-3-2-0-rc2-has-been-released. (called from /Users/danielstjules/GitHub/Markus/config/environment.rb:9)

There was some discussion about `/vendor` before - had a decision been made on how we'd like to handle things? It's not something that needs immediate action.